DISINTEGRATION OF SOILS. 113 



It is evident that plants also, by producing carbonic acid during 

 their decay, and by means of the acids which exude from their 

 roots in the living state, contribute no less powerfully to destroy 

 the coherence of rocks. Next to the action of air, water, and 

 change of temperature, plants themselves are the most powerful 

 agents in effecting the disintegration of rocks. 



Air, water, and ohange of temperature prepare the different 

 species of rocks for yielding to plants their alkalies. A soil ex- 

 posed for centuries to all the influences which effect the disinte- 

 gration of rocks, but from which the alkalies, thus rendered 

 soluble, have not been removed, will be able to afford, during 

 many years, the means of nourishment to vegetables requiring a 

 considerable amount of alkalies for their growth ; but it must 

 gradually become exhausted, unless those alkalies which have 

 been removed are again replaced ; a period, therefore, will arrive 

 when it will be necessary to expose it from time to time to a 

 further disintegration, in order to obtain a new supply of soluble 

 alkalies. For, small as is the quantity of alkali essential to 

 plants, it is nevertheless quite indispensable for their perfect de- 

 velopment. But when one or more years have elapsed without 

 the removal of any alkalies from the soil, a new harvest may be 

 expected. 



The first colonists of Virginia found a soil similar to that 

 mentioned above ; harvests of wheat and tobacco were obtained 

 for a century from one and the same field, without the aid of 

 manure ; but now whole districts are abandoned and converted 

 into unfruitful pasture-land, which without manure produces 

 neither wheat nor tobacco. From every acre of this land there 

 were removed in the space of one hundred years 12,000 lbs. of 

 alkalies in leaves, grain, and straw ; it became unfruitful there- 

 fore, because it was deprived of every particle of alkali fit for 

 assimilation, and because that which was rendered soluble again 

 in the space of one year was not sufficient to satisfy the demands 

 of the plants. Almost all the cultivated land in Europe is in 

 this condition ; fallow is the term applied to land left at rest for 

 further disintegration. It is the greatest possible mistake to sup. 

 pose that the temporary diminution of fertility in a soil is owing 



